Wednesday 25 August 2010 16.05 EDT
First published on Wednesday 25 August 2010 16.05 EDT

The private life of an MI6 officer who was murdered at his home close to the UK's foreign intelligence service headquarters is being investigated by Scotland Yard detectives. The body of Gareth Williams was stuffed in a holdall and dumped in his bath.

Williams is believed to have been killed two weeks ago. His body was badly decomposed when his two-bedroom, top floor flat in Pimlico, central London, was searched by police after colleagues raised the alarm. A postmortem examination found that he had not been stabbed, as had been reported earlier, but the cause of death has yet to be established. Further tests, including analysis of his blood for evidence of drugs and alcohol, will now take place.

When police entered his flat they found Williams's body, a mobile phone and several sim cards laid out in a ritual manner.

The 31-year-old was on secondment to MI6 HQ from the government's eavesdropping and communications security centre GCHQ in Cheltenham, Gloucestershire. He is believed to have been advising MI6 on secret communication techniques. Many technical staff at Cheltenham spend time advising MI6 and MI5.

Informed Whitehall sources said GCHQ "boffins" – of all ages – were subjected to particular pressure in their private life because of the nature of their specialist knowledge and skills and the secrecy surrounding them. Williams would have been subject to "developed vetting" before being employed at GCHQ.

Officers from Scotland Yard's homicide and serious crime command are leading the inquiry, which sources said pointed to the fact that Williams's intelligence work was not believed to be linked to his death. "National security" seemed to be the least likely of any issues connected with the death, said a Whitehall official.

Police went to the flat "following reports that the occupant had not been seen for some time", they said. His body was removed from the flat on Tuesday evening and forensics experts were still searching the premises late this afternoon.

Detectives are examining Williams's lifestyle and recent movements as part of the investigation and will question his colleagues in London and Cheltenham.

His family live in Holyhead, Anglesey, north Wales. Today a police officer from London was seen leaving the home after formally telling Williams's parents, Ian and Ellen, and sister Ceri the news.

Cath Stanley, 65, a neighbour and retired Merseyside police superintendent, said: "Gareth was a smashing lad and really into his cycling. His father was called 'Mr Fitness' round here because he was a keep-fit fanatic. I don't even think either of them drank. I think they were teetotal. Gareth and his dad used to cycle round here together, in Lycra kit and helmets."

Williams's uncle, William Hughes, told BBC News: "It was terrible when I got the phone call yesterday morning. Just didn't know what to believe really."

Asked about Williams's career, his uncle called his nephew "very, very talented in his work. We don't know what he was doing – never spoke about it".

In June 2001, before joining MI6, Williams dropped out of studying a masters in advanced mathematics at St Catharine's College, Cambridge, after failing an exam. He moved to Cheltenham, where he began his career at GCHQ, which employs 5,500 people.

His former landlady, Jenny Elliott, 71, said he had lived in a flat in her house near Cheltenham for 10 years. He told her he worked at the Foreign Office.

She said he had been due to move back into her house in 10 days after spending a year in London. "He came to live with me in a flat on the side of my house when he started to work at the Foreign Office. He was with me for about 10½ years and then he moved out a year ago. He was due to come back to me on 3 September. He rang me and said he would be back then. He said, 'Can I come back?' and I said sure. I hadn't heard anything else until a lady from the Foreign Office called at six o'clock on Monday night to say that they hadn't had a sighting or a whereabouts and had we heard anything. She said she was the Foreign Office's head of employee assistance.

"This awful thing is happening and Gareth was a lovely man, very well-mannered and very likeable. He was very clever and had been to Cambridge and had a very important job at the Foreign Office. Although he didn't belong to me, I was quite proud of him. It's like losing one of my own children. We expected to hear from him any minute. He was coming back here to work.

"When he went to London, I said, 'You won't like it up there, it's not like the countryside. But he said he wanted a change of scene. Those were his words."

Today, police cordons remained around 36 Alderney Street, a mid-terrace property in a street of five-storey Victorian townhouses in Pimlico. A community support officer stood outside. The wealthy area, a few blocks from Victoria station, is home to bankers and politicians, including two former home secretaries, Michael Howard and Leon Brittan.

Neighbours, who described Williams as "extremely friendly", said they had been told by investigators the murder may have happened up to two weeks ago.

Laura Houghton, 30, said she had met Williams once at the entrance hall of the flats about a year ago. She said: "His windows were always shut and curtains were often closed. I could never tell if anyone was in. It was strange that we never saw him come and go. I just assumed he worked away."

A body was brought out of the building on a stretcher under a maroon sheet at about 9.20pm on Tuesday. Police were looking at CCTV footage in the area. A nearby hotel, the Windermere, has a CCTV camera that covers the junction of Warwick Way and Alderney Street, and it is understood they have made it available.

The property where Williams lived is divided into four flats and is registered to a private company, New Rodina, with an offshore address in the British Virgin Islands. Public documents revealed several current and former residents of the block have links to London and Cheltenham. One Frenchman who lived there between 2005 and 2006 is an expert in global satellite positioning, radio communications and high-sensitivity antennas.

The deputy prime minister, Nick Clegg, has been informed of the death and any security implications.

A spokesperson for the Foreign Office refused to give any details about Williams: "It is longstanding government policy not to confirm or deny the identity of any individual working for the intelligence agencies. This is a police matter."

MI6 officer Gareth Williams was filmed on CCTV at Holland Park tube station in west London nine days before his body was found in a holdall in his Pimlico flat. The footage has been released by Scotland Yard